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I am not answering the question, but I have a tip on how to use USB drives.

GRUB4DOS has no USB driver. But you may boot from a virtual (DOS) floppy that has a USB driver in, like this:

map --mem (hd0,0)/floppy_with_usb_support.img (fd0)
map --hook
chainloader (fd0)+1
rootnoverify (fd0)
boot